The invention disclosed and claimed herein generally pertains to a method for identifying packet type in a system wherein data is transmitted over a link by means of packets. More particularly, the invention pertains to a method of the above type wherein the headers of at least some of the packets are subjected to 0-byte compression prior to transmission, so that the packets are header free and packet type must be identified implicitly. Even more particularly, the invention pertains to a method of the above type which is particularly well adapted for use with narrow band links, such as cellular and wireless links.
Due to the great success of the Internet, it has become a challenging task to make use of the Internet Protocol (IP) over many different kinds of links. However, because the headers of the IPs are rather large, it can be difficult to use IPs in connection with narrow band links such as wireless and cellular links. For example, for ordinary speech data transported by the ordinary protocols (IP, UDP, RTP) the header may represent about 70% of the packet. This results in a very inefficient usage of the link.
The term header compression (HC) refers to the art of minimizing the necessary bandwidth for information carried in headers on a per-hop basis over point-to-point links. HC techniques in general have a more than 10-year-old history within the Internet community, and several commonly used protocols exist such as RFC 1144 (hereinafter “VJ”, set forth in a document prepared by Van Jacobson, et al, entitled “Compressing TCP/IP Headers for Low-Speed Serial Links,” IETF RFC 1144, IETF Network Working Group, February 1990); RFC 2507 (hereinafter “IPHC”, set forth in a document by Degermark, Nordgren and Pink, entitled “IP Header Compression,” IETF RFC 2507, IETF Network Working Group, February 1999); and RFC 2508 (hereinafter “CRTP”, set forth in a document by Casner and Van Jacobson, entitled “Compressing IP/UDP/RTP Headers for Low-Speed Serial Links”, IETF RFC 2508, IETF Network Working Group, February 1999).
Header compression takes advantage of the fact that some fields in the headers do not change within a flow, or else change with small and/or predictable values. Accordingly, header compression schemes send static information only initially, while fields that change are sent with their absolute values or as differences from packet-to-packet. Completely random information must, of course, be sent without any compression at all. The header compression scheme can usually be realized as a state machine, and a challenging task associated with compression is keeping the compressor and decompressor states (called contexts) consistent with each other, while keeping the header overhead as low as possible.
It will be readily apparent that header compression is very important in making Voice over IP (VoIP) over Wireless (VoIPoW) an economically feasible alternative to circuit switched voice. Header compression solutions for this purpose have been developed by the Robust Header Compression (ROHC) Working Group of IETF. These solutions are set forth in a document authored by Bormann, et al., entitled “Robust Header Compression,” IETF RFC 3095, July 2001. The ROHC RTP header compression scheme has been designed to efficiently compress the IP/UDP/RTP headers over an arbitrary link layer. Except for negotiation, ROHC RTP compression requires only that framing and error detection be provided by the link layer, while all other functionality, such as packet type identification, is handled by the ROHC scheme itself.
Recent efforts in header compression further include the creation of 0-byte header compression schemes for IP/UDP/RTP packets. Schemes of this type are described, for example, in a document by Jonsson, et al., entitled “A Link-Layer Assisted ROHC Profile for IP/UDP/RTP,” RFC 3242 April 2002 (hereinafter “LLA”). Such schemes use functionality provided by the lower layer to increase compression efficiency by completely eliminating the header for most packets during normal operation. These header free packets can be sent when some of their information may be inferred by the functionality provided by the assisting layer. This information includes the field of the header that identifies packet type.
Even though header free packets may now be provided by schemes such as LLA, a packet type identifier is still always needed in all headers compressed using the ROHC technique, thereby requiring a minimal header size of one octet. Thus, the provision of packet type identification without the explicit use of additional bits over the air interface becomes problematic, and the use of 0-byte header compression schemes will be hindered until this problem can be solved. To completely eliminate the header, a packet type identifier must therefore be provided through other means, to be used when a “free” link layer bit is not available and when the transmission of additional bits is not possible.